How to use Gazebo mimic joints plugin?

asked 2020-02-09 18:30:28 -0600

facureyes gravatar image

[TL;DR] I dont know how to implement and use mimic_joint_plugin in my project. I would appreciate some help or an example so I can look at it.

Im developing a quadruped robot, with the particularity that it only has two motors per leg. It has a parallel linkage in the elbow, so the 3rd link is always orthogonal to the ground.

So I made my urdf file using mimic joints, and the legs work as I want to. The problem came when I tried to do the same thing on Gazebo. Gazebo 7 (Im working with ROS KK and Ubuntu 16.04) doesnt support mimic joints, so I need to use mimic joint plugin (because I dont want to make a differente sdf file, so my Gazebo model can integrate with everything else in ROS). I kind of follow this tutorial but I still cant make it work.

In my opinion, I havent fully understand how Im supposed to define the mimic joints with the pluging. This is what I have done on cuadrubot_control.yaml:

    type: joint_state_controller/JointStateController
    publish_rate: 50

    type: effort_controllers/JointPositionController
    joint: leg1_joint1
    pid: {p: 70.0, i: 60.0, d: 15.0}

Where I repeat legX_jointX_position_controller for all my not mimic joints.

Then on my cuadrubot_control.launch file:

<!-- Load joint controller configurations from YAML file to parameter server -->
  <rosparam file="$(find cuadrubot_control)/config/gazebo/cuadrubot_control.yaml"       

<!-- load the controllers -->
  <node name="controller_spawner" pkg="controller_manager" type="spawner" respawn="false"
    output="screen" ns="/cuadrubot" args="legX_jointX_position_controller 
                                          joint_state_controller "/>

As far as I know, this is right.

In my macro.xacro file I have done this for each mimic joint (joint3 of each leg):

<xacro:macro name="joints" params="name origin_rpy front side">
  <joint name="${name}_joint1" type="revolute">
    <parent link="base_link"/>
    <child link="${name}_link1"/>
    <limit lower="-${pi/4}" upper ="${pi/4}" effort = "10" velocity = "10"/>
    <origin xyz="${ancho_chasis/2*front} ${largo_chasis/2*side} 0" rpy="${origin_rpy}" />
    <axis xyz="0 0 1" />
  <joint name="${name}_joint2" type="revolute">
    <parent link="${name}_link1"/>
    <child link="${name}_link2"/>
    <limit lower="-${pi/2}" upper ="${pi/2}" effort = "10" velocity = "10"/>
    <origin xyz= "${largo_link1} 0 0" rpy= "${pi/2} 0 0" />
    <axis xyz="0 0 1" />
  <joint name="${name}_joint3" type="revolute">
    <parent link="${name}_link2"/>
    <child link="${name}_link3"/>
    <limit lower="-${pi/2}" upper="${pi/2}" effort="10" velocity="10"/>
    <mimic joint="${name}_joint2" multiplier="-1" offset="-${pi/2}" />
    <origin xyz="${largo_link2} 0 0" rpy="0 0 ${pi/2}" />
    <axis xyz="0 0 1" />
  <transmission name="${name}_joint1_trans">
    <joint name="${name}_joint1">
    <actuator name="${name}_joint1_motor">
      <mechanicalReduction>1</mechanicalReduction> <!-- 26 -->
  <transmission name="${name}_joint2_trans">
  <joint name="${name}_joint2">
    <hardwareInterface>hardware_interface/EffortJointInterface</hardwareInterface> ...
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